The present invention relates to light emitting diodes and related structures (lasers and UV emitting diodes) that are formed from Group III nitride compounds using silicon carbide or other suitable substrates such as sapphire or GaN for the devices.
The present invention is an improved buffer structure for Group III nitride devices, particularly photonic devices, formed on silicon carbide substrates. An excellent background (and more) discussion on photonic devices is set forth in Sze, PHYSICS OF SEMICONDUCTOR DEVICES, Chapter 12, page 681ff (1981), John Wiley & Sons, Inc. As set forth therein, there are several categories of photonic devices. The present invention will be described in terms of light emitting diodes, with the understanding that the invention can be incorporated as well with the other types of devices.
The use of Group III nitrides as active layers in light emitting devices is set forth in the co-pending incorporated references as well as other sources in this art. Because to date the bulk growth of Group III nitrides has proven difficult and has not reached widespread commercial availability, Group III nitride devices are typically formed on other substrates of which the two most common in commercial versions are silicon carbide (SiC) and sapphire (Al2O3). Although the present invention can be incorporated with sapphire substrates, the present specification will speak mainly in terms of silicon carbide as being the preferred and most convenient way to describe both the background and the invention itself.
In many circumstances silicon carbide is preferred for a substrate material because it can be conductively doped. Silicon carbide differs from sapphire in such respect because sapphire cannot be conductively doped. Accordingly, devices incorporated on sapphire must have some sort of laterally based structure to accommodate the respective ohmic contacts that inject current through the device. In contrast, because silicon carbide is conductive, the ohmic contacts can be placed at opposite ends of the device, an orientation that is referred to in the art as “vertical.” In many circumstances and devices, such a vertical orientation offers design and production advantages. Silicon carbide also offers a number of physical and electronic advantages, such as a wide bandgap, a high melting point, a high thermal conductivity, a high breakdown electric field, and other factors well understood in this art that make it an attractive candidate material for substrates and devices.
As a different compound from gallium nitride or any other Group III nitride, however, the crystal lattice parameters of silicon carbide are different from the crystal lattice parameters of the Group III nitrides, taken either individually (i.e. binary compounds), or in their various ternary or quaternary combinations. Thus, when Group III nitride layers are formed into devices on silicon carbide substrates, some accommodation must be made to help provide a transition between the crystal structure of silicon carbide and the crystal structure of the Group III nitrides. As is well understood by those of ordinary skill in this art, lattice mismatches can cause defects (some of which are referred to as “dislocations”) that propagate through the respective layers of a device and degrade its performance. If severe enough, such defects can render the device inoperable.
Accordingly, transition structures referred to as “buffers” (or “buffer layers”) are almost always included in Group III nitride devices on SiC substrates.
Buffer structures of various types have been disclosed in the art. Exemplary ones are set forth in commonly assigned U.S. Pat. Nos. 5,523,589 and 5,393,993, as well as in the co-pending applications incorporated herein by reference. Because of the continued interest in and importance of Group III nitrides in photonic devices however, there remains a need and interest in obtaining buffer structures and buffer growth techniques that offer constant improvement in this area.